The RX 7700 XT is a newer RDNA 3 architecture, the advantages it has over the RX 6800 are:
AV1 encode / decode, that's a better more efficient video codex in the sense that you get a better image quality at a smaller file size for video recording, streaming and if you do any video editing / rendering in apps that support it.
RT performance per core is quite a bit higher on RDNA 3, for example a quick google shows the RX 6800 scores 7400 in Port Royal while the RX 7700 XT scores 8950, that's 21% higher, that score is actually 9% higher that the RTX 3070 which scores 8200, a 4060 Ti also scores 8200, so not bad.
So far as i'm aware while the RX 6000 series will support FSR 3.0, i don't think it will support FSR 3.1, see the difference between FSR 3.1 and FSR 3.0.
RDNA 3 has an AI engine, RDNA 2 does not, that could be important for future features and possibly why RDNA 2 (RX 6000) may not support FSR 3.1.
FSR 2.2 / 3.0
FSR 3.1
The RX 6800 has 16 GB of VRam vs only 12 GB on the 7700 XT, 16 GB is a nice solid amount, but 12 GB is not bad, at 1440P there isn't anything that's going to be a problem for that, you know.... its not like its 8GB, a year or two in the future that might change, you might have to make some small image quality compromises, or it might be great for a couple of years or more, it's impossible to predict but in the long run 16GB is a much safer bet.
I was recently in the market for a GPU and thought long and hard about all of this myself, what i ended up buying was an RX 7800 XT but i had decided on the RX 7700 XT if it was going to be between the RX 6800 and it, the benefits outweigh the 12 GB VRam, IMO.
I have the RX 7800 XT version of this RX 7700 XT, its the one i recommend, despite its price and small size it actually a VERY good cooler, its a good quality card.
Order SAPPHIRE PULSE AMD Radeon™ RX 7700 XT Gaming Graphics Card with 12GB GDDR6, AMD RDNA™ 3 architecture now online and benefit from fast delivery.
www.overclockers.co.uk
If you want the fancy one i do think this is reasonably priced.
Order SAPPHIRE NITRO+ AMD Radeon™ RX 7700 XT Gaming Graphics Card with 12GB GDDR6, AMD RDNA™ 3 architecture now online and benefit from fast delivery.
www.overclockers.co.uk
Others please chime in